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Old 03-13-2011, 02:39 AM   #1
Feb 2010
Lincoln, United Kingdom
Posts: 949
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I originally thought a 150 amp main breaker box could have hypothetically 10 15 amp circuits in it, i.e., 150 total amps.

Then I looked at mine, and I noticed if I count up all the breakers, there's like 400 potential amps in there. Obviously the NEC and common sense doesn't count on everything being turned on at the same time so it's not a practical problem.

That being said, what is the formula for figuring out how many breakers and at what amperage I can put in there? Can someone guide me to the code?

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Old 03-13-2011, 05:09 AM   #2
EdWort's Avatar
Jul 2006
Bee Cave, Texas
Posts: 11,912
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Usually, you buy a breaker box at a rated amperage. They do make 400 amp breaker boxes, so do you know if yours is rated at 150 amps?

My house has a 200 amp service and the brewhaus has 100 amp service. Each breaker box breakers do not add up to the total service rating.

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Old 03-13-2011, 05:35 AM   #3
Feb 2011
Westphalia, Ks
Posts: 111
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*The amperage rating of all the branch circuit breakers in your panel are not necessarily additive...the power drawn on the main service is determined by the actual service load calculations (a math system of demand factors and derating that lowers the final amperage number)....and for that reason - if you add up all the listed breaker ratings in the typical 100 or 200 amp'll almost always find that number far exceeds the main breaker size (as you noted)...and this is common and Code acceptable. For example, the typical 100 amp panel may have 1 - 50 or 60 amp double pole (stove), 2 - 30 amp double poles (dryer and water heater)...and a half dozen or more 15 and 20 amp single pole breakers...and never draw more then 70 amps max at any given time....and most often only a few amps at most times. (If I were to put my clamp-on amp-probe on my main service wires right now...I'd estimate I'd find I'm only drawing 2 or 3 amps at this hour....if even that).

**To list all the demand and derating formula's for service load calculations here would exceed the file size limits of this site...but if you have access to a 2005 NEC Code book (most library's have a copy in their reference section)'ll find the tables and examples we use in it for calculating main breaker size ,etc...and that will help you get a better understanding of how and why the sum of the branch circuit breakers always seems to add up to more then the main breaker....often much more.

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Old 03-20-2011, 09:51 PM   #4
May 2010
Posts: 33

the breakers must match the wires that come off it. If you have 14 gauge wires you don't want to put a 30 amp breaker, the breaker is supposed to be the "weakest link".

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Old 03-20-2011, 10:53 PM   #5
Sparky's Avatar
Nov 2008
Muir Beach, California
Posts: 291
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Originally Posted by badmajon View Post
That being said, what is the formula for figuring out how many breakers and at what amperage I can put in there? Can someone guide me to the code?
Two answers:
First, size the breakers according to the load in the house (and wire size). Branch circuits.

Second, you need to do a load calculation to ensure the right size (total amps). I don't have time to get into the nuances of the calc's, much of it based off sections 210 and 220 in the NEC. If you want to study, there is a great book by Charles Miller, "Illustrated Guide to the NEC, based on 2011 NEC." It has the load calc sheet.

There is additional things to do like balancing your panel, etc.

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